upon a suitable Kähler transformation.
The inflation region corresponds to \(\varphi \rightarrow \infty \) , or equivalently \(|y|\rightarrow 1\) , for vanishing \(C\)  or \(z\) , where the scalar potential is approximately constant \(\propto \mu ^2\) . The main problem of this model is that the sgoldstino \(C\)  (or \(z\) ) is unstable (tachyonic) during inflation [1]}. Stabilisation of the runaway direction can be achieved by adding for instance a quartic (or higher order) term in the Kähler potential which obviously modifies the theory beyond the minimal supersymmetrisation of \(R^2\) . Alternatively, one can parametrise the stabilisation mechanism in a model independent way by integrating out the heavy sgoldstino replacing \(C\)  with the nilpotent goldstino superfield satisfying the constraint \(C^2=0\)  [2]}. Here, we will follow a different approach based on the derived effective field theory of free-fermionic string models.
